Propylene-based multi-layer films are widely used in packaging applications, such as pouches for dry food mixes, pet foods, snack foods, and seeds. Such multi-layer films must have the ability to resist peeling at the interface between the layers of the multi-layered film.
Typical multi-layered films include at least a core layer, typically comprised of polypropylene (PP), an outer layer typically known as a skin layer, typically comprised of polar polymers, and a tie layer located intermediate the core layer and the skin layer. Conventional tie layers are mostly maleic anhydride (MAH) grafted polypropylene (PP-g-MAH) homopolymers, copolymers, or combinations of PP-g-MAH with non-grafted polyolefins. In general, these maleated adhesive tie layers lack peel resistance at their interfaces with other adjacent layers. This is because they are poor in stress dissipation and reactivity due to their high chain rigidity and interfacial tension. Many applications, such as laminates of toothpaste tubes, require much stronger interfacial strength than these conventional film compositions can provide.
U.S. Pat. No. 6,919,407 discloses a film reported to have enhanced machine direction (MD) tear properties. The film has at least one polymer layer comprising at least 50 wt % propylene and at least 5 wt % ethylene and/or one or more unsaturated comonomers, such as C4-C20 alpha olefins and dienes.
U.S. Pat. No. 4,561,920 describes a method of producing bi-axially oriented oxygen and moisture barrier film comprising co-extruding at least one layer of polyolefin material (polyethylene or polypropylene or ethylene-propylene copolymers), at least one layer of ethylene vinyl alcohol and at least one adhesive layer. The adhesive layer used in the patent is Admer QF500B, which is a maleic anhydride-modified polypropylene.
U.S. Pat. No. 4,650,721 describes a process for forming a firmly bonded co-extruded film combination. The process comprises forming an intimate combination of polypropylene and a maleic anhydride-modified olefin copolymer and forming a film of at least one layer of the intimate combination and an ethylene vinyl alcohol copolymer. The adhesion is effected by orienting the film. The maleic anhydride grafted polyolefin is a maleic anhydride grafted polypropylene homopolymer and has a softening point of 157° C.
U.S. Pat. No. 4,880,706 describes a multilayer co-extruded film having been bi-axially oriented and comprising a five-layer structure, ABCBA. A is polyethylene, polypropylene or copolymers of ethylene and higher alpha-olefins. B is an adhesive layer and C is a layer of an ethylene vinyl alcohol copolymer. The adhesive layer includes an ethylene-vinyl acetate copolymer and a maleic acid modified olefin polymer.
U.S. Pat. No. 6,472,081 discloses an oriented multilayer film combination including (1) a propylene homopolymer or copolymer core layer; (2) a metallizable skin layer, including a material selected from an ethylene-vinyl alcohol copolymer (EVOH), a polyvinyl alcohol (PVOH), and polyester; and (3) an adhesive tie layer made from a material selected from a maleic anhydride modified polypropylene homopolymer or copolymer, a high density polyethylene (HDPE), and an ethylene-vinyl acetate (EVA) copolymer. The adhesive tie layer is disposed between the core layer and the skin layer.
European Patent No. 1 614 699 provides a polypropylene composite film layer with 0-95 wt % of crystalline polypropylene and 5-100 wt % of propylene/1-butene random copolymer. The random copolymer contains: 60-90 mol % of propylene-derived units and 10-40 mol % of 1-butene-derived units, and has a molecular weight distribution (Mw/Mn) of 1 to 3, and a melting point of 40-120° C.
European Patent No. 1 541 602 discloses a propylene polymer with a propylene main chain having a stereo-block structure containing an isotactic block and a side chain containing a carboxylic acid group, an acid anhydride group or a carboxylic acid ester group. The propylene polymer main chain is obtainable by using a single site catalyst wherein the single site catalyst is a C1-symmetric ansa-metallocene compound having a transition metal-containing bridge group.
Other patents and publications of relevance to this disclosure include: European Patents 1,017,729; 629,631 and 629,632.
It would be very useful to the films industry to produce films that include tie layers with improved peel strength, greater elasticity, greater energy dissipation and lower interfacial tension with ethylene-based polar polymers and polar modified polyolefins of the skin layer, such as ethylene vinyl alcohol (EVOH).